The present invention relates to a process and apparatus for regulating the filter plate contact pressure in a plate filter press comprising a hydraulic closing cylinder means.
In filter presses, the filter plates and, in plate-and-frame presses, the frames, are respectively arranged between two filter plates and are mounted on a horizontal or vertical guide means to be slidable or movable. In order to initiate a filtration process, the filter plates are moved together by a hydraulic closing cylinder unit to form a closed filter plate pack. In this process, a closing pressure must be exerted such that the filter chambers located between the filter plates are reliably sealed along the marginal edges thereof. During filtration, the slurry, fed into the filter chambers under a frequently high pressure, exerts an expansion pressure which is directed toward a spreading of the filter plate pack and such expansion pressure must be compensated for by a corresponding increase in the closing pressure. In the same manner, it is necessary in a membrane filter press to compensate for pressure of the press medium or, in filter presses, when the filter cake is washed out to compensate for the pressure of the wash water, by raising the closing pressure.
During the filtration process, the filter cake pressing step and the filter cake washing step, a high closing pressure is exerted on the filter plate pack such that this pressure, upon the elimination of the slurry, press medium, resulting in such a filter plate contact pressure that the filter plates or the optionally intermediately located filter frames, filter cloths, or press membranes would be destroyed.
In order to avoid the above noted problem, it has been proposed to regulate the closing pressure of the hydraulic closing cylinder unit in dependence upon an internal pressure respectively prevailing the in the filter plate pack. For this purpose, measurements are conducted to determine the hydraulic pressure of the closing cylinder unit and the pressures of the slurry, of the press medium, and of the wash water, with the pressure differential between the hydraulic closing pressure and the respectively exerted slurry, press medium, or wash water pressure being respectively determined in a difference-generating means, and with the closing pressure being controlled for maintaining a specific desired pressure differential. A disadvantage of this proposed arrangement resides in the fact that a considerable expense is necessary in the structural components since it is necessary to provide several pressure sensing units such as gauges, a selection unit with respect to the internal pressure to be compensated for in a particular case, i.e., slurry, press medium, or wash water, and a differential pressure generator. Furthermore, the differential pressure utilized therein as the control variable is in a only indirect connection with the actual contact pressure of the filter plates and, for this reason, this proposed type of regulation or control is inaccurate.
The aim underlying the present invention essentially resides in providing a process and apparatus for regulating the filter plate contact pressure which is inexpensive yet achieves a very high accuracy. In accordance with advantageous features of the present invention, the control variable utilized is the compression force prevailing in the filter plate pack or the compression of the entire filter plate pack or the compression of at least one part exposed to the compression pressure ambient in the filter plate pack. Moreover, the hydraulic pressure of the closing cylinder unit is regulated with a view toward maintaining a desired compression value.
In order to measure the compression force, it is possible, in accordance with the present invention, to employ, for example, dynamometer cells arranged at the end plate of the filter press. The compression force or the compression of the entire filter plate pack or also the compression of a portion exposed to the compression load is in direct correlation with the contact pressure exerted on the filter plates so that, with the use of such control variable, a very accurate monitoring is possible.
According to the present invention, the compression can be determined by utilizing only a single sensing device independently of the fact of whether the closing pressure acts against the pressure of the slurry, the pressure medium, or the wash water, whereby it is possible to minimize the cost for structural components to achieve the regulating method.
In accordance with advantageous features of the process of the present invention, the compression of the entire filter plate pack can be determined by a sensor detecting a position of a head plate of the filter press. For this purpose, it is possible to employ optical sensors, ultrasonic sensors, pneumatic sensors, microswitches, or non-contacting operating proximity switches, or the like.
Advantageously, according to the present invention, the compression of at least one filter plate, or a frame in a plate-and-frame press, or of a separate deformation plate exposed to the compression pressure may be measured by, for example, strain gauges. By virtue of the utilization of strain gauges, even minimum compression displacements can be accurately detected and reproduced.
In accordance with still further features of the process of the present invention, the compression pressure of the filter plate pack may be determined by one or more piezoelectric sensor means inserted in the filter plate pack or interposed between an end plate and the next-proximate filter plate. In this manner, it is possible to determine, at a relatively low cost with regard to structural components, a control variable for the effective filter plate contact pressure.
In order to carry out the process of the present invention wherein the compression is determined by strain gauges, a device may be provided wherein the strain gauges are arranged along narrow sides of at least one of the filter plates or one of the frames or the deformation plate with electrical output signals of the strain gauges being transmitted directly or through a measured value converter to an actual value-desired value comparator of a controller. A hydraulic pressure of the closing cylinder unit would then be regulated in dependence upon a correcting instruction from the controller. The strain gauges may be mounted directly to the narrow side of the filter plate of the frame or of the deformation plate by, for example, a suitable adhesive such as, glue. This construction is extremely simple in particular if the filter plate and, respectively, the frame or the deformation plate is made of a relatively strong compressible material such as, for example, an elastically compressible synthetic resin.
However, it is also possible in accordance with the present invention to arrange, along the narrow sides of the filter plate or the frame or of the deformation plate, one or more measuring elements which are deformed in case of compression of the filter plate or of the frame or of the deformation plate and to attach the strain gauges at the deformation zones of the measuring elements.
The measuring elements, if suitably constructed, undergo or experience a stronger deformation than that of the filter plate or similar members carrying such elements so that thereby the measuring sensitivity can be considerably increased. As can be appreciated, an increase in the measuring sensitivity by up to two powers of ten can be achieved by providing, according to the present invention, that the measuring element include an approximately U-shaped or arcuate member of, for example, steel, with a ridge of the U-shaped or arcuate member being oriented perpendicularly to the primary plane of the filter plates and the two bases being rigidly attached to a narrow side of the filter plate or the like. The ridge of the measuring element is subjected to a sagging upon compression of the filter plate carrying the same, there being present, on a ridge side facing away from the filter plate pack, a compression deformation zone. Preferably, according to the present invention, the strain gauges are mounted at the elongation deformation zone as well as at the compression deformation zone.
Additionally, according to the present invention, the strain gauges are arranged and the associated evaluating circuit is constructed in such a manner that temperature compensation is taken into account. The measures to be taken for such a temperature compensation are conventional in connection with measuring units comprising strain gauges and, consequently, a detailed discussion thereof is deemed unnecessary.
The above and other objects, advantages, and features of the present invention will become more apparent from the following description when taken in connection with the accompanying drawings which show several embodiments of the present invention.